Aluminum alloy films for use in display devices are mainly used as electrodes and wiring materials. Examples of the electrodes and wiring materials include gate, source, and drain electrodes for a thin film transistor and a wiring material in a liquid crystal display (LCD); gate, source, and drain electrodes for a thin film transistor and a wiring material in an organic EL (OLED); cathode and gate electrodes and a wiring material in a field emission display (FED); an anode electrode and a wiring material in a vacuum fluorescent display (VFD); an address electrode and a wiring material in a plasma display panel (PDP); and a back electrode in an inorganic EL.
Hereinafter, while a liquid crystal display is representatively described as a liquid crystal display device, the present invention is not limited thereto.
Large-sized liquid crystal displays are widely used as main display devices because of advancement in low power consumption technology. Liquid crystal displays having a size of more than 100 inches are now commercialized. There are various types of liquid crystal displays having different operating principles. Among them, active-matrix liquid crystal displays including thin film transistors (hereinafter, referred to as “TFTs”) used for the switching of pixels are most-widely used because they have high-precision image qualities and can display fast moving images. In liquid crystal displays required to have lower power consumption and higher switching speeds of pixels, TFTs including semiconductor layers composed of polycrystalline silicon and continuous grain silicon are used.
For example, active-matrix liquid crystal displays having amorphous silicon include TFTs serving as switching elements, pixel electrodes comprising of a conductive oxide film, and wiring such as scan lines and signal lines. The scan and signal lines are electrically connected to pixel electrodes. Wiring materials constituting scan lines and signal lines are formed of Al-based alloy thin films such as an Al—Ni alloy (Patent literatures 1-5 for example). For displays having polycrystalline silicon, refractory metal such as Mo are used for wiring materials constituting scan lines while Al-based alloy thin films such as an Al—Ni alloy are adopted as wiring materials constituting signal lines.
The structure of a core portion of a TFT substrate including a semiconductor layer composed of polycrystalline silicon is described below with reference to FIG. 1. FIG. 1 illustrates a structure after depositing various kinds of wiring and pattering them.
As illustrated in FIG. 1, a scan line 4 and a polycrystalline silicon layer 2, a semiconductor layer, are arranged on a glass substrate 1. A part of the scan line 4 functions as a gate electrode 5 that controls the on/off state of a TFT. The gate electrode 5 is electrically insulated with a gate insulating film 7 (a silicon nitride film for example). A semiconductor polycrystalline silicon layer 2 is arranged as a channel layer on the gate insulating film 7. The polycrystalline silicon layer 2 is connected to a part of signal line 10 such as a source electrode 8 and a drain electrode 9 with a low-resistance polycrystalline silicon layer 3 and has electrical conductivity. The drain electrode 9 is connected to transparent electrode 12 comprising such as indium tin oxide (ITO). The low-resistance polycrystalline silicon layer 3 is formed, after fabricating the scam line 4, by ion-implantation of elements such as phosphorus or boron followed by activation heat treatment at high temperature of about 450° C. to 600° C.
As just described the scan line 4 could be subjected to high temperature of about 450° C. to 600° C. The Al-based alloys for use in wiring of TFTs disclosed in the patent literatures 1-5 are, however, insufficient in terms of heat resistance at high temperatures. Their heatproof temperature is 350° C. at the highest. Instead of the Al-based alloys, refractory metals such as Mo and Mo-based alloys which are excellent in high temperature heat resistance are being used. The refractory metals such as Mo and Mo-based metals have, however, high electrical resistance.
A process to remove native oxides formed on the surface of polycrystalline silicon layer 3 and via hole 11 may be carried out prior to connecting source electrode 8 and drain electrode 9 which are part of signal line 10 to polycrystalline silicon layer 3 of low resistance. This is due to a fact that TFT characteristics is deteriorated by the formation of the native oxides which increase contact resistance of source electrode 8 and drain electrode 9 with polycrystalline silicon layer 3. Wet cleaning with about 1 percent hydrofluoric acid (dilute hydrofluoric acid) solution is generally conducted. Due to poor resistance to hydrofluoric acid, conventional Al-based alloy thin films had a problem of dissolving by cleaning using hydrofluoric acid solution of polycrystalline silicon layer 3 and via hole 11 in the TFT structure illustrated in FIG. 1.